ESTRO 35 Abstract book

S78 ESTRO 35 2016 _____________________________________________________________________________________________________

Purpose or Objective: Voluntary moderately deep inspiration breath hold (vmDIBH) reduces the heart dose for radiotherapy of left-sided breast cancer patients. For locoregional breast cancer patients, the application of vmDIBH requires high reproducibility to assure the absence of gap or overlay between tangential breast fields and supraclavicular irradiation fields. In this study we present a simple and fast visual method to quantify movement around the junction of the tangential and supraclavicular fields. The simple method is evaluated by testing the target volume reproducibility using two consecutive CT-scans during vmDIBH. Heart position reproducibility is assessed as well, with the resulting dosimetric consequences. Material and Methods: For 80 consecutive breast cancer patients cranial-caudal (CC) displacement around the clavicle was quantified between five vmDIBHs. This was done in the CT room, before obtaining the planning CT scan. Intersecting CT laser lines were marked on tape and the maximum displacement was measured. This tape was positioned midclavicularly, with the horizontal laser lines on the junction line. For 19 patients who would be irradiated locoregionally, a second CT scan was additionally acquired. The CC displacement of the left clavicle between the two breath holds was quantified by contouring the clavicle in both CT scans, and rigid registration of the two volumes in ProSoma (v.3.3.266, Medcom, Darmstadt, Germany) virtual simulation software. The heart was delineated in both CT scans, excluding the great vessels. The two volumes were registered in ProSoma to measure CC, left-right (LR) and anterior-posterior (AP) displacements. Influence of the heart displacement on dosimetry was measured by superimposing the contoured heart volume of the second CT scan onto the treatment planning CT scan and calculating mean heart dose. Results: Results of the tape test show a mean CC displacement of 3.3 mm (range 0.5-8.0 mm) for the midclavicular region. For the two breath hold CT scans mean CC clavicle displacement was 1.1 mm (range 0.1 - 2.8 mm). The measured CC displacements of the tape test were for all 19 locoregional patients larger than measured with CT. Mean difference in contoured heart volume was 3.7% (range 0.5 - 11.2%). Mean heart dose differed on average 0.12 Gy (range 0.01 - 0.38 Gy), where planned mean heart dose varied between 0.59 and 3.58 Gy. Mean heart displacement was 1.7 mm (range 0-4.7 mm) CC, 1.5 mm (range 0.1-4.2 mm) AP and 1.9 mm (range 0.1-6.9 mm) LR.

Gantry angle started at 181 degree, stopped at 35 degree, and the 240-300 degree was set to be avoidance sector. Two half arcs were for left-side breast and axillary nodes. Gantry angle started at 153 degree, stopped at 282 degree, and the 0-90 angle was set to be avoidance sector. Gantry angle started at 282 degree, stopped at 161 degree, and the 0-90 angle was set to be avoidance sector. One quarter arc was for internal mammary node. Gantry angle started at 179 degree, stopped at 270 degree, and the 60-120 angle was set to be avoidance sector. The dose-volume-histogram were evaluated the target homogeneity and conformity and normal tissue tolerance dose.

Results: MVMAT has significantly (p-value = 0.031) decreased right-side breast dose (V5Gy (%) = 39.9 ± 7.6), and has significantly (p-value = 0.005) decreased right lung dose (V5Gy (%) = 23.6 ± 7.3). Slightly less heart and left lung dose are found for MVMAT (heart V10Gy (%) = 45.7 ± 17.4, left lung V10Gy (%) = 48.6 ± 3.4) than in VMAT (heart V10Gy(%)=55.7 ± 19.9, left lung V10Gy(%)=53.4 ± 5.1). MVMAT for advanced left-side breast cancer retains target homogeneity and coverage when compared to VMAT.

Conclusion: MVMAT is suitable for advanced left-side breast cancer treatment. It retains target homogeneity and coverage and decreases the dose of right breast and right lung. OC-0168 A simple visual test is adequate for testing vmDIBH reproducibility in locoregional breast cancer L.A. Den Otter 1 , T.T. Nuver 1 , M. Boerhof 1 , H.H. Kolkman- Nijland 1 , W.J. Schoevers 1 , K. Muller 1 , A.W.H. Minken 1

1 Radiotherapiegroep,

Radiotherapy,

Deventer,

The

Netherlands

Conclusion: A simple visual test is a good surrogate for CT scans in analyzing vmDIBH reproducibility. We showed that vmDIBH is reproducible with minimal gap or overlay between